This invention relates to methods for analyzing the chemical composition of coal. More particularly, the invention relates to a non-destructive method employing photothermal spectroscopic techniques to detect coal constituents including, in a preferred form of the invention, the detection of variations of chemical composition at successive microscopic regions of a coal sample. The United States government has rights in this invention pursuant to contract number DE-AC-03-76SF00098 between the U.S. Department of Energy and the University of California.
The morphological structure of coal typically exhibits a mixture of chemically distinct inclusions or macerals such as vitrinite, exinite, micronite and fusinite among others. Such macerals may be present in varying proportions and in varying sizes in coals obtained from different mines or different sites within a particular mine.
Different types of maceral have different chemical compositions and significantly different chemical behaviors. Consquently, analysis of the maceral composition of particular coal samples can be very useful in connection with selecting particular coals for particular usages. For example, the maceral composition is a major determinant of the rank or heat producing capability of a coal. As a further example, the macerals vitrinite and exinite are readily susceptible to liquifaction while micrinite and fusinite are not. Significant physical properties, such as hardness for example, are also determined in part by the types, sizes and distribution of the various macerals within the coal.
As macerals are typically microscopic inclusions in coal, identifying the chemical compositions and preferably the sizes and distributions of particular macerals requires techniques capable of resolving the desired information at the microscopic level. This cannot be accomplished by conventional methods of chemical analysis unless an undesirably complex and lengthy series of exacting operations are performed. In particular, the coal sample must be physically separated into minute portions each of which must then be separately analyzed and the resulting data must then be correlated with the original locations of the minute portions in the sample.
Thus the processing and utilization of coal can be greatly facilitated by a more convenient and preferably non-destructive method for analyzing the chemical composition of coal on a microscopic scale. Preferably, the method should detect variations of chemical composition between very minute successive regions including regions at different depths to enable evaluation of the maceral composition of a three dimensional portion of a coal sample.